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Abstract:

A method, system, and computer program product for timing driven routing
in a design of an integrated circuit (IC) are provided in the
illustrative embodiments. A router application executing in a data
processing system performs a pre-global routing optimization of the
design. A plurality of wirelength target constraints are set on a
plurality of subsets of a set of nets in the design. Global routing is
performed on the design. The design is adjusted using wires placed in the
design during the global routing. A priority is assigned to each net in
the set of nets. Detailed routing is performed on the design.

Claims:

1. A computer implemented method for timing driven routing in a design of
an integrated circuit (IC), the method comprising: performing, using a
router application executing in a data processing system, a pre-global
routing optimization of the design; setting a plurality of wirelength
target constraints on a plurality of subsets of a set of nets in the
design; performing global routing on the design; adjusting the design
using wires placed in the design during the global routing; assigning a
priority to each net in the set of nets; and performing detailed routing
on the design.

2. The computer implemented method of claim 1, wherein the design is an
design obtained after a placement operation, wherein the pre-global
routing optimization produces a pre-global routing optimized design,
wherein setting the plurality of wirelength target constraints uses the
pre-global routing optimized design, wherein the global routing uses the
pre-global routing optimized design and the plurality of wirelength
target constraints to produce a global routed design, wherein the
adjusting uses the global routed design to produce an adjusted global
routed design, and wherein the detailed routing uses the adjusted global
routed design and the priority associated with each net in the set of
nets to produce a detailed routed design.

3. The computer implemented method of claim 1, wherein performing the
pre-global routing optimization further comprises: performing a pin
density spreading operation on the design; selecting a net from the set
of nets whose timing characteristic has exceeded a threshold responsive
to the pin density spreading operation; and adjusting the timing
characteristic of the net in-place such that a cell to which the net
belongs does not change position.

4. The computer implemented method of claim 3, wherein adjusting the
timing characteristic of the net the in-place is accomplished using one
of (i) threshold voltage tuning, and (ii) gate resizing, further
comprising: performing a second global routing on the design including a
change from the adjusting; determining, following the second global
routing, whether the design has deteriorated responsive to the change;
and undoing the change.

5. The computer implemented method of claim 1, wherein the setting the
plurality of wirelength target constraints further comprises:
categorizing into a plurality of categories, each net in the set of nets
according to a timing criticality and a wirelength associated with each
net; and associating with each category in the plurality of categories a
wirelength target constraint.

6. The computer implemented method of claim 5, wherein setting the
plurality of wirelength target constraints further comprises: assigning,
to a net whose wirelength in the design is less than a threshold
wirelength, a wirelength target constraint that is more stringent as
compared to a second wirelength target constraint assigned to a second
net whose wirelength in the design is more than the threshold wirelength;
and assigning, to a third net whose slack is smaller than a slack
threshold, a third wirelength target constraint that is more stringent as
compared to a fourth wirelength target constraint assigned to a fourth
net when the third net is more timing critical than the fourth net.

7. The computer implemented method of claim 1, wherein the adjusting the
design using the wires placed in the design during global routing further
comprises: selecting, following the global routing, a net from the set of
nets whose timing characteristic exceeds a threshold; and performing,
causing a change in the design, a timing optimization on the net using a
subset of the wires placed in the design during the global routing.

8. The computer implemented method of claim 7, further comprising:
performing a second global routing; determining, following the second
global routing, whether the design has deteriorated responsive to the
change; and undoing the change.

9. The computer implemented method of claim 1, wherein assigning the
priority further comprises: categorizing into a plurality of categories,
the nets in the set of nets according to a wirelength of each net;
adjusting a timing characteristic of each net in a category from the
plurality of categories; measuring a slew rate of each net in the
category; sorting the nets within the category according to each net's
slew rate, forming a sorted order of nets; and assigning a priority to
each net in the category according to the net's position in the sorted
order of nets.

10. The computer implemented method of claim 9, wherein a priority
assigned to a net in the set of nets is used during a subsequent routing,
and wherein a first net with a higher priority as compared to a priority
of a second net is routed before the second net.

11. A computer usable program product comprising a computer usable
storage medium including computer usable code for timing driven routing
in a design of an integrated circuit (IC), the computer usable code
comprising: computer usable code for performing, using a router
application executing in a data processing system, a pre-global routing
optimization of the design; computer usable code for setting a plurality
of wirelength target constraints on a plurality of subsets of a set of
nets in the design; computer usable code for performing global routing on
the design; computer usable code for adjusting the design using wires
placed in the design during the global routing; computer usable code for
assigning a priority to each net in the set of nets; and computer usable
code for performing detailed routing on the design.

12. The computer usable program product of claim 11, wherein the design
is an design obtained after a placement operation, wherein the pre-global
routing optimization produces a pre-global routing optimized design,
wherein setting the plurality of wirelength target constraints uses the
pre-global routing optimized design, wherein the global routing uses the
pre-global routing optimized design and the plurality of wirelength
target constraints to produce a global routed design, wherein the
adjusting uses the global routed design to produce an adjusted global
routed design, and wherein the detailed routing uses the adjusted global
routed design and the priority associated with each net in the set of
nets to produce a detailed routed design.

13. The computer usable program product of claim 11, wherein performing
the pre-global routing optimization further comprises: computer usable
code for performing a pin density spreading operation on the design;
computer usable code for selecting a net from the set of nets whose
timing characteristic has exceeded a threshold responsive to the pin
density spreading operation; and computer usable code for adjusting the
timing characteristic of the net in-place such that a cell to which the
net belongs does not change position.

14. The computer usable program product of claim 13, wherein adjusting
the timing characteristic of the net the in-place is accomplished using
one of (i) threshold voltage tuning, and (ii) gate resizing, further
comprising: computer usable code for performing a second global routing
on the design including a change from the adjusting; computer usable code
for determining, following the second global routing, whether the design
has deteriorated responsive to the change; and computer usable code for
undoing the change.

15. The computer usable program product of claim 11, wherein the setting
the plurality of wirelength target constraints further comprises:
computer usable code for categorizing into a plurality of categories,
each net in the set of nets according to a timing criticality and a
wirelength associated with each net; and computer usable code for
associating with each category in the plurality of categories a
wirelength target constraint.

16. The computer usable program product of claim 15, wherein setting the
plurality of wirelength target constraints further comprises: computer
usable code for assigning, to a net whose wirelength in the design is
less than a threshold wirelength, a wirelength target constraint that is
more stringent as compared to a second wirelength target constraint
assigned to a second net whose wirelength in the design is more than the
threshold wirelength; and computer usable code for assigning, to a third
net whose slack is smaller than a slack threshold, a third wirelength
target constraint that is more stringent as compared to a fourth
wirelength target constraint assigned to a fourth net when the third net
is more timing critical than the fourth net.

17. The computer usable program product of claim 11, wherein the
adjusting the design using the wires placed in the design during global
routing further comprises: computer usable code for selecting, following
the global routing, a net from the set of nets whose timing
characteristic exceeds a threshold; and computer usable code for
performing, causing a change in the design, a timing optimization on the
net using a subset of the wires placed in the design during the global
routing.

18. The computer usable program product of claim 11, wherein the computer
usable code is stored in a computer readable storage medium in a data
processing system, and wherein the computer usable code is transferred
over a network from a remote data processing system.

19. The computer usable program product of claim 11, wherein the computer
usable code is stored in a computer readable storage medium in a server
data processing system, and wherein the computer usable code is
downloaded over a network to a remote data processing system for use in a
computer readable storage medium associated with the remote data
processing system.

20. A data processing system for timing driven routing in a design of an
integrated circuit (IC), the data processing system comprising: a storage
device including a storage medium, wherein the storage device stores
computer usable program code; and a processor, wherein the processor
executes the computer usable program code, and wherein the computer
usable program code comprises: computer usable code for performing, using
a router application executing in a data processing system, a pre-global
routing optimization of the design; computer usable code for setting a
plurality of wirelength target constraints on a plurality of subsets of a
set of nets in the design; computer usable code for performing global
routing on the design; computer usable code for adjusting the design
using wires placed in the design during the global routing; computer
usable code for assigning a priority to each net in the set of nets; and
computer usable code for performing detailed routing on the design.

Description:

BACKGROUND

[0001] 1. Technical Field

[0002] The present invention relates generally to a method, system, and
computer program product for designing an integrated circuit. More
particularly, the present invention relates to a method, system, and
computer program product for improving the design of the integrated
circuit (IC) by using timing driven routing.

[0003] 2. Description of the Related Art

[0004] Modern day electronics include components that use integrated
circuits. Integrated circuits are electronic circuits formed using
Silicon as a substrate and by adding impurities to form solid-state
electronic devices, such as transistors, diodes, and resistors. Commonly
known as a "chip", an integrated circuit is generally encased in hard
plastic. The components in modern day electronics generally appear to be
rectangular black plastic pellets with connector pins protruding from the
plastic encasement.

[0005] Circuit designers use a variety of software tools to design
electronic circuits that accomplish an intended task. For example, a
digital circuit may be designed to accept digital inputs, perform some
computation, and produce a digital output. An analog circuit may be
designed to accept analog signals, manipulate the analog signals, such as
my amplifying, filtering, or mixing the signals, and produce an analog or
digital output. Generally, any type of circuit can be designed as an IC.

[0006] Logical synthesis, physical synthesis, and generation of a routed
and timing closed design are some of the functions of an IC design
software tool. Logical synthesis is the process of designing the logical
operation that is to be achieved by a circuit. Physical synthesis is the
mapping, translating, or integration of that logical synthesis to the
physical design components, such as logic gate and buffer circuits.
Routing and timing closed design is the design produced by adjusting the
wire routings in and component placements in a design so that the design
meets certain design criteria such as delay or slew of signals, or
wirelength restrictions.

[0007] The software tools used for designing ICs produce, manipulate, or
otherwise work with the circuit layout at very small scales. Some of the
components that such a tool may manipulate may only measure tens of
nanometer across when formed in Silicon. The designs produced and
manipulated using these software tools are complex, often including
millions of such components interconnected to form an intended electronic
circuitry. An interconnected group of components is called a net.

[0008] The software tools manipulate these components at the components
level, or blocks of components level. A block of components is also known
as a global cell, or g-cell. A g-cell in an IC design is a portion of the
IC design. One way of identifying g-cells in an IC design is to overlay a
grid of imaginary vertical and horizontal lines on the design, and
deeming each portion of the IC design bound by horizontal and vertical
lines as a g-cell. The horizontal or vertical lines bounding a g-cell are
called cut-lines.

[0009] Imposing such a grid on an IC design abstracts the global routing
problem away from the actual wire implementation and gives a more
mathematical representation of the task. A net may span one or more
g-cells and may cross several cut lines.

[0010] An IC design software tool can, among other functions, manipulate
cells, or interconnect components of one cell with components of other
cells, such as to form nets. These cells are different from g-cells in
that these cells are the actual logic components, such as the
semiconductor gates. The interconnects between components are called
wires. A wire is a connection between parts of electronic components, and
is formed using a metallic material that conducts electricity.

[0011] Placement problem is the problem of placing the cells of a chip
such that the design meets all the design parameters of the chip. Routing
is the process of connecting the pins after placement. In other words,
placement results in a rendering of the components of various cells as
being located in certain positions in the design, whereas routing results
in a rendering of how the metal layers would be populated with that
placement.

[0012] A wire can be designed to take any one of the several available
paths in a design. Placement of a wire on a certain path, or track, is a
part of routing.

[0013] A layer is typically designated to accommodate wires of a certain
width (wire code). Generally, the wider the wire width of a layer or
higher the height of a layer, the faster the signal propagation speed for
the net routed on that layer. Faster layers, to with, layers with larger
wire widths and wire height, can accommodate fewer components or nets as
compared to slower layers with narrower wire widths.

[0014] A router is a component of an IC design tool that performs the
routing function. Once the placement component--known as a placer--has
performed the placement function, the router attempts to connect the
wires without causing congestion. For example, if a design parameter
calls for no more than five wires in a given area, the router attempts to
honor that restriction in configuring the wiring. Such limitations on the
wiring are a type of design constraints and are called congestion
constraints. Other types of design constraints may include, for example,
blocked areas--cell areas where wires may not be routed.

[0015] A global router divides the routing region into g-cells and
attempts to route nets through the g-cells such that no g-cell overflows
its capacity. Global routing is the process of connecting a g-cell to
other g-cells.

[0016] After global routing, wires must be assigned to actual tracks
within each tile, followed by detail routing, which must connect each
global route to the actual pin shape on the cell. Another type of
router--known as the detailed router--performs the detailed routing. The
global and detailed routing produced during the design process is
collectively referred to as "routing" and is usually further modified
during optimization of the design.

SUMMARY

[0017] The illustrative embodiments provide a method, system, and computer
program product for timing driven routing in integrated circuit design.
An embodiment performs, using a router application executing in a data
processing system, a pre-global routing optimization of the design. The
embodiment sets a plurality of wirelength target constraints on a
plurality of subsets of a set of nets in the design. The embodiment
performs global routing on the design. The embodiment adjusts the design
using wires placed in the design during the global routing. The
embodiment assigns a priority to each net in the set of nets. The
embodiment performs detailed routing on the design.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018] The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself, however, as well as a
preferred mode of use, further objectives and advantages thereof, will
best be understood by reference to the following detailed description of
an illustrative embodiment when read in conjunction with the accompanying
drawings, wherein:

[0019] FIG. 1 depicts a pictorial representation of a network of data
processing systems in which illustrative embodiments may be implemented;

[0020] FIG. 2 depicts a block diagram of a data processing system in which
illustrative embodiments may be implemented;

[0021] FIG. 3 depicts a flowchart of an overall timing driven routing
process in accordance with an illustrative embodiment;

[0022] FIG. 4 depicts a flowchart of a pre-global routing optimization
process in accordance with an illustrative embodiment;

[0023] FIG. 5 depicts a flowchart of an example process for setting
wirelength targets in accordance with an illustrative embodiment;

[0024] FIG. 6 depicts a flowchart of an example process for performing
post global routing optimization using the global routing wires resulting
from global routing in accordance with an illustrative embodiment; and

[0025] FIG. 7 depicts a flowchart of a process for assigning priorities to
nets in a design according to an illustrative embodiment.

DETAILED DESCRIPTION

[0026] One of the functions of the placement and optimization step in IC
design is to minimize the wirelengths of the nets in the design. The
longer the wirelength of a net, the longer the delay in the net. Many
wirelength optimization algorithms are available. Steiner tree algorithm
is a well known algorithm used by many placement tools for forming the
nets in an IC design.

[0027] Presently, the placement step in an IC design process produces a
design that includes nets with their timing optimized within desirable
timing thresholds. The invention recognizes that routing such an
optimized design in the IC design process often deteriorates, severely in
some cases, the timing characteristics of the post placement optimized
nets.

[0028] The invention recognizes that presently used routers are not aware
of the criticality of the nets present in the optimized design they are
routing. A net is critical if a change in a characteristic of the net,
such as delay in the net, violates a parameter or specification of the
design.

[0029] Therefore, when a router routes a previously optimized design, the
timing and the overall figure of merit of the design are often
deteriorated. In one example routing, the Steiner optimized timing metric
of -0.17 nano second was observed to deteriorate to -9.73 nano second
after routing. For that example routing, the overall figure of merit of
the design deteriorated from -5.4 after Steiner optimization to -16422
after routing.

[0030] The illustrative embodiments used to describe the invention
generally address and solve the above-described problems and other
problems related to routing an IC design. The illustrative embodiments
provide a method, system, and computer program product for improved IC
design using timing driven routing.

[0031] While some embodiments are described with respect to each net in a
design, an implementation may use an embodiment with respect to less than
all the nets in the design without departing the scope of the invention.
For example, an implementation may some but not all the nets in the
manner of an embodiment without departing the scope of the invention.

[0032] The illustrative embodiments are described with respect to certain
ICs or circuits only as examples. Such descriptions are not intended to
be limiting on the invention. For example, an illustrative embodiment
described with respect to a microprocessor design can be implemented with
respect to any other IC design whose routing can be improved by using an
embodiment.

[0033] The illustrative embodiments are described with respect to certain
data, data structures, file-systems, file names, directories, and paths
only as examples. Such descriptions are not intended to be limiting on
the invention. For example, an illustrative embodiment described with
respect to a local application name and path can be implemented as an
application on a remote path within the scope of the invention.

[0034] Furthermore, the illustrative embodiments may be implemented with
respect to any type of data, data source, or access to a data source over
a data network. Any type of data storage device may provide the data to
an embodiment of the invention, either locally at a data processing
system or over a data network, within the scope of the invention.

[0035] The illustrative embodiments are described using specific code,
designs, architectures, layouts, schematics, and tools only as examples
and are not limiting on the illustrative embodiments. Furthermore, the
illustrative embodiments are described in some instances using particular
software, tools, and data processing environments only as an example for
the clarity of the description. The illustrative embodiments may be used
in conjunction with other comparable or similarly purposed structures,
systems, applications, or architectures. An illustrative embodiment may
be implemented in hardware, software, or a combination thereof.

[0036] The examples in this disclosure are used only for the clarity of
the description and are not limiting on the illustrative embodiments.
Additional data, operations, actions, tasks, activities, and
manipulations will be conceivable from this disclosure and the same are
contemplated within the scope of the illustrative embodiments.

[0037] Any advantages listed herein are only examples and are not intended
to be limiting on the illustrative embodiments. Additional or different
advantages may be realized by specific illustrative embodiments.
Furthermore, a particular illustrative embodiment may have some, all, or
none of the advantages listed above.

[0038] With reference to the figures and in particular with reference to
FIGS. 1 and 2, these figures are example diagrams of data processing
environments in which illustrative embodiments may be implemented. FIGS.
1 and 2 are only examples and are not intended to assert or imply any
limitation with regard to the environments in which different embodiments
may be implemented. A particular implementation may make many
modifications to the depicted environments based on the following
description.

[0039] FIG. 1 depicts a pictorial representation of a network of data
processing systems in which illustrative embodiments may be implemented.
Data processing environment 100 is a network of computers in which the
illustrative embodiments may be implemented. Data processing environment
100 includes network 102. Network 102 is the medium used to provide
communications links between various devices and computers connected
together within data processing environment 100. Network 102 may include
connections, such as wire, wireless communication links, or fiber optic
cables. Server 104 and server 106 couple to network 102 along with
storage unit 108. Software applications may execute on any computer in
data processing environment 100.

[0040] In addition, clients 110, 112, and 114 couple to network 102. A
data processing system, such as server 104 or 106, or client 110, 112, or
114 may contain data and may have software applications or software tools
executing thereon.

[0041] Any data processing system, such as server 104, may include router
105 that may be improved using an embodiment. Application 107 may be any
combination of hardware and software usable for implementing an
embodiment of the invention such that the embodiment is usable with
router 105 for creating an improved routing.

[0043] In the depicted example, server 104 may provide data, such as boot
files, operating system images, and applications to clients 110, 112, and
114. Clients 110, 112, and 114 may be clients to server 104 in this
example. Clients 110, 112, 114, or some combination thereof, may include
their own data, boot files, operating system images, and applications.
Data processing environment 100 may include additional servers, clients,
and other devices that are not shown.

[0044] In the depicted example, data processing environment 100 may be the
Internet. Network 102 may represent a collection of networks and gateways
that use the Transmission Control Protocol/Internet Protocol (TCP/IP) and
other protocols to communicate with one another. At the heart of the
Internet is a backbone of data communication links between major nodes or
host computers, including thousands of commercial, governmental,
educational, and other computer systems that route data and messages. Of
course, data processing environment 100 also may be implemented as a
number of different types of networks, such as for example, an intranet,
a local area network (LAN), or a wide area network (WAN). FIG. 1 is
intended as an example, and not as an architectural limitation for the
different illustrative embodiments.

[0045] Among other uses, data processing environment 100 may be used for
implementing a client-server environment in which the illustrative
embodiments may be implemented. A client-server environment enables
software applications and data to be distributed across a network such
that an application functions by using the interactivity between a client
data processing system and a server data processing system. Data
processing environment 100 may also employ a service oriented
architecture where interoperable software components distributed across a
network may be packaged together as coherent business applications.

[0046] With reference to FIG. 2, this figure depicts a block diagram of a
data processing system in which illustrative embodiments may be
implemented. Data processing system 200 is an example of a computer, such
as server 104 or client 110 in FIG. 1, in which computer usable program
code or instructions implementing the processes of the illustrative
embodiments may be located for the illustrative embodiments.

[0047] In the depicted example, data processing system 200 employs a hub
architecture including North Bridge and memory controller hub (NB/MCH)
202 and south bridge and input/output (I/O) controller hub (SB/ICH) 204.
Processing unit 206, main memory 208, and graphics processor 210 are
coupled to north bridge and memory controller hub (NB/MCH) 202.
Processing unit 206 may contain one or more processors and may be
implemented using one or more heterogeneous processor systems. Graphics
processor 210 may be coupled to the NB/MCH through an accelerated
graphics port (AGP) in certain implementations.

[0049] An operating system runs on processing unit 206. The operating
system coordinates and provides control of various components within data
processing system 200 in FIG. 2. The operating system may be a
commercially available operating system such as Microsoft®
Windows® (Microsoft and Windows are trademarks of Microsoft
Corporation in the United States, other countries, or both), or
Linux® (Linux is a trademark of Linus Torvalds in the United States,
other countries, or both). An object oriented programming system, such as
the Java® programming system, may run in conjunction with the
operating system and provides calls to the operating system from Java®
programs or applications executing on data processing system 200 (Java
and all Java-based trademarks and logos are trademarks or registered
trademarks of Oracle and/or its affiliates).

[0050] Program instructions for the operating system, the object-oriented
programming system, the processes of the illustrative embodiments, and
applications or programs are located on storage devices, such as hard
disk drive 226, and may be loaded into a memory, such as, for example,
main memory 208, read only memory 224, or one or more peripheral devices,
for execution by processing unit 206. Program instructions may also be
stored permanently in non-volatile memory and either loaded from there or
executed in place. For example, the synthesized program according to an
embodiment can be stored in non-volatile memory and loaded from there
into DRAM.

[0051] The hardware in FIGS. 1-2 may vary depending on the implementation.
Other internal hardware or peripheral devices, such as flash memory,
equivalent non-volatile memory, or optical disk drives and the like, may
be used in addition to or in place of the hardware depicted in FIGS. 1-2.
In addition, the processes of the illustrative embodiments may be applied
to a multiprocessor data processing system.

[0052] In some illustrative examples, data processing system 200 may be a
personal digital assistant (PDA), which is generally configured with
flash memory to provide non-volatile memory for storing operating system
files and/or user-generated data. A bus system may comprise one or more
buses, such as a system bus, an I/O bus, and a PCI bus. Of course, the
bus system may be implemented using any type of communications fabric or
architecture that provides for a transfer of data between different
components or devices attached to the fabric or architecture.

[0053] A communications unit may include one or more devices used to
transmit and receive data, such as a modem or a network adapter. A memory
may be, for example, main memory 208 or a cache, such as the cache found
in north bridge and memory controller hub 202. A processing unit may
include one or more processors or CPUs.

[0054] The depicted examples in FIGS. 1-2 and above-described examples are
not meant to imply architectural limitations. For example, data
processing system 200 also may be a tablet computer, laptop computer, or
telephone device in addition to taking the form of a PDA.

[0055] With reference to FIG. 3, this figure depicts a flowchart of an
overall timing driven routing process in accordance with an illustrative
embodiment. Process 300 may be implemented as an improvement in router
105 in FIG. 1, or as application 107 usable to improve the operation of
router 105 in FIG. 1.

[0056] Presently, following the timing optimization after placement, the
design process performs global routing followed by the detailed routing,
which deteriorate the timing in the routed design as described earlier.
Process 300 begins by receiving an optimized design, such as a timing
optimized design after the placement step in the design process (step
302). Process 300 performs a pre-global routing optimization, such as
process 400 in FIG. 4 (step 304).

[0057] Process 300 sets target wirelength constraints for a set of nets,
such as according to process 500 in FIG. 5 (step 306). A set of nets is
one or more nets. Process 300 then performs the global routing (step
308).

[0060] Several steps of process 300 are described in detail in subsequent
figures as indicated. Furthermore, process 300 may repeat one or more
steps in an iterative manner to incrementally improve the global routed
design before performing the detailed routing. For example, as shown,
process 300 may perform more than one pass through steps 306-312 before
reaching the detailed design step.

[0061] Additionally, an iteration may undo a previous iteration if the
incremental modifications of the previous iteration fail to meet a design
specification. Process 300 may also perform certain steps in an order
other than as shown in FIG. 3. For example, in one embodiment, process
300 may assign net priorities before the global routing of step 308, and
set the target wirelengths afterwards. In another embodiment, process 300
may set the target wirelengths and assign net priorities before the
global routing of step 308.

[0062] With reference to FIG. 4, this figure depicts a flowchart of a
pre-global routing optimization process in accordance with an
illustrative embodiment. Process 400 may be implemented as step 304 in
process 300 in FIG. 3.

[0063] Process 400 begins by performing pin density spreading to improving
the routability of the optimized design from placement (step 402).
Process 400 selects a net whose timing has deteriorated beyond a
threshold as a result of the pin density spreading (step 404).

[0064] For the selected net, process 400 performs in-place optimization of
the design after the pin density spreading so that the timing
optimization of the placement step is minimally disturbed (step 406). In
other words, process 400 performs the optimization within a cell such
that the cell does not have to be moved to a new location.

[0065] For example, process 400 may not insert buffers for the
optimization of step 406 because buffer additional may cause a cell to
move to a different location, thereby disturbing the wirelengths of nets
that cross that cell's boundaries. Process 400 may perform the
optimization of step 406 by tuning the threshold voltage of components or
resizing the gates in-place, or by using other in-place timing
adjustments.

[0066] Process 400 evaluates whether the design has deteriorated as a
result of the in-place optimization (step 408). Process 400 may perform
global routing (not shown in FIG. 4, but see a comparable step 606 in
FIG. 6) using the optimized design resulting from step 406 to make the
determination of step 408. In terms of time and computing resources,
global routing is relatively inexpensive as compared to detailed routing.
Therefore, global routing can be run even after incremental changes to
determine the suitability of the incremental change, and whether to
reject that change.

[0067] For example, the in-place optimization of step 406 may cause
another design criterion, such as the delay of another net or a slew rate
of a signal at a location in the design, to exceed a threshold condition.
If the design has been deteriorated, such as when a design criterion is
violated ("Yes" path of step 408), process 400 backs out or removes the
modifications performed during the previous execution of the optimization
step 406 (step 410). Process 400 returns to step 404 to select the same
net and try a different in-place optimization or to select a different
net and perform in-place optimization on the different net.

[0068] The in-place optimization of step 406 may improve the timing of the
selected net and may maintain other design criteria within specified
thresholds. Thus, if the design shows an improvement as a result of the
in-place optimization, or at least no deterioration beyond a threshold
("No" path of step 408), process 400 determines whether more nets remain
to be optimized using in-place timing optimization in a similar manner
(step 412). If more nets are to be optimized for timing corrections after
the pin density spreading ("Yes" path of step 412), process 400 returns
to step 404 and iterates through steps 404-412 as described above with
another net. If no more nets are to be optimized ("No" path of step 412),
process 400 ends thereafter.

[0069] With reference to FIG. 5, this figure depicts a flowchart of an
example process for setting wirelength targets in accordance with an
illustrative embodiment. Process 500 may be implemented as step 306 in
process 300 in FIG. 3.

[0070] A wirelength target for a net is a threshold wirelength for the
net. In other words, the wirelength target limits the maximum length of
wire the router is allowed to use when routing the net.

[0071] Process 500 begins by categorizing the nets in the design into
categories according to each net's timing criticality and length (step
502).

[0072] In operation, step 502 includes first dividing the nets in the
design into two broad categories--critical nets and non-critical nets
using a slack threshold. Slack is a timing characteristic of a net
representing the difference between a signal's actual arrival time at a
point and the desired arrival time of the signal at that point.

[0073] Step 502 also categorizes the nets in step 502 according to their
wirelengths. Any number of length thresholds may be defined to categorize
the nets according to their wirelengths. For example, a single length
threshold will enable classifying the nets into two categories--those
that are shorter than the threshold (short), and those that are longer
than the threshold (long). Similarly, defining two thresholds for lengths
allows for three categories of nets--shorter than the first threshold
(short), between the two thresholds (medium), and longer than the second
threshold (long). In a similar manner, an implementation using nine
thresholds can categorize the nets into ten categories according to their
wirelengths. For the clarity of the description and not as a limitation
on the invention, the operations of process 500 are explained using the
short, medium, and long wirelength categories.

[0075] For example, short nets are typically driven by smaller power
sources and normally tend to be more timing critical than the longer
nets. Accordingly, step 504 may define the target constraints differently
for the different categories of nets.

[0076] Using the three-category example, in one embodiment, the wirelength
target constraint for each category of nets may be defined as not to
exceed the Steiner length of the net plus a specified fixed length. For
example, the short nets may not exceed the Steiner length+150
micrometers, the medium nets may not exceed the Steiner length+300
micrometers, and the long nets may not exceed the Steiner length+450
micrometers.

[0077] In another embodiment, the wirelength target constraint for each
category of nets may be defined as not to exceed a multiple of the net's
Steiner length. For example, the short nets may not exceed one hundred
and twenty percent of the net's Steiner length, the medium nets may not
exceed two hundred percent of the net's Steiner length, and the long nets
may not exceed three hundred percent of the net's Steiner length.

[0078] In another embodiment, the wirelength target constraints for some
categories of nets may be defined differently than others. For example, a
short net may not to exceed the Steiner length of the net plus a
specified fixed length. The medium nets may not exceed two hundred
percent of the net's Steiner length, and the long nets may not exceed
three hundred percent of the net's Steiner length.

[0079] In another embodiment, the wirelength target constraints for some
categories of nets may be defined differently than others based on
multiple conditions or considerations. For example, a short net may not
to exceed the Steiner length of the net plus a specified fixed length.
Those nets in the medium category whose slack is greater than a slack
threshold may not exceed two hundred percent of the net's Steiner length.
Those nets in the long category whose slack is greater than the slack
threshold may not exceed three hundred percent of the net's Steiner
length. Furthermore, those nets in the medium or long categories whose
slack is less than the slack threshold may not exceed one hundred and
thirty percent of the net's Steiner length if the net is in top 0.05
percent according to the timing criticality. Those nets in the medium or
long categories whose slack is less than the slack threshold may not
exceed one hundred and fifty percent of the net's Steiner length if the
net is next 8 percent of the nets according to the timing criticality.
Those nets in the medium or long categories whose slack is less than the
slack threshold may not exceed one hundred and eighty percent of the
net's Steiner length if the net is next 15 percent of the nets according
to the timing criticality.

[0080] The above embodiments describe various example ways of categorizing
the nets in a design and for defining wirelength targets for those
categories. These examples are described only for the clarity of the
disclosure and not as a limitation on the invention. Any number targets
may be defined in a similar manner to suit a particular implementation.
Those of ordinary skill in the art will be able to conceive many other
ways of defining the wirelength thresholds from this disclosure, and the
same are contemplated within the scope of the invention.

[0081] With reference to FIG. 6, this figure depicts a flowchart of an
example process for performing post global routing optimization using the
global routing wires resulting from global routing in accordance with an
illustrative embodiment. Process 600 may be implemented as step 310 in
FIG. 3.

[0082] Process 600 starts by selecting a net from the global routed design
where the net's timing is unacceptable, for example, where the net's
delay exceeds a threshold (step 602). Process 600 performs any suitable
timing optimization of the net using the wires placed by the global
router during the global routing (step 604). The timing optimization of
the net in step 604 may use any suitable optimization technique or a
combination thereof, including but not limited to a combination of
threshold voltage tuning, repowering a gate, or buffering the net.

[0083] Process 600 may (optionally) perform global routing again after the
optimization of step 604 (step 606). As noted above, global routing is
relatively inexpensive as compared to detailed routing. Therefore process
600 may chose to perform global routing at step 606 after the incremental
optimization of step 604 to determine the suitability of the changes made
to the design in step 604.

[0084] Process 600 determines, such as by evaluating the design after
performing global routing of step 606, whether the optimization of step
604 has resulted in an deterioration of the design (step 608). If the
design has deteriorated ("Yes" path of step 608), process 600 may chose
to undo the changes made by the previous iteration through step 604 (step
610). Step 608 may determine whether the design has deteriorated as
compared to the previous design by, for example, determining whether a
design parameter that was not violated in the previous design is violated
after optimization.

[0085] If the design has improved, or at least not deteriorated beyond a
threshold ("No" path of step 608), process 600 determines whether more
nets exist in the design with unacceptable timing characteristics (step
612). If more nets with unacceptable timing characteristics exist ("Yes"
path of step 612), process 600 returns to step 604. If no more nets with
unacceptable timing characteristics exist ("No" path of step 612),
process 600 ends thereafter.

[0086] An example way of determining whether a net has an unacceptable
timing characteristic is to compare the net's timing violation to a
threshold for timing violation and consider the net as acceptable if the
timing of the net, even though in violation, does not exceed the
threshold for timing violation, because some timing violations can be
corrected even after the global routing phase of the design has been
completed. While process 600 has been described with respect to timing
violations and timing characteristics, process 600 is adaptable to use
other characteristics of a net, such as slew or capacitance
characteristics, in a similar manner without departing the scope of the
invention.

[0087] With reference to FIG. 7, this figure depicts a flowchart of a
process for assigning priorities to nets in a design according to an
illustrative embodiment. Process 700 may be implemented as step 312 in
process 300 in FIG. 3.

[0088] Process 700 begins by categorizing a set of nets in a design
according to their wirelengths (step 702). Process 700 selects a category
(step 704). Process 700 adjusts the timing of the nets in that category
by a factor (step 706). For example, process 700 may select a category of
short nets in step 704. In step 706, process 700 may, for example, by
simulation, increase the resistance, capacitance, inductance, or a
combination thereof, of the short nets ten folds.

[0089] As another example, process 700 may select a category of medium or
long nets in step 704. In step 706, process 700 may, for example, by
simulation, increase the resistance, capacitance, inductance, or a
combination thereof, of the medium or long nets by one hundred and forty
percent. By adjusting the timing in this manner, process 700 can expose
the weak gates in the nets in the selected category for further
modifications.

[0090] Process 700 measures the slew rate or ratio of the nets in the
selected category after the timing adjustment of step 706 (step 708).
Process 700 sorts the nets in the selected category by their measured
slew rates (step 710).

[0091] Process 700 assigns priorities to the nets in the selected category
according to the nets' positions in the sorted order of step 710 (step
712). For example, in a selected category of short nets, process 700 may
assign the nets in the top 0.05 percent of the sorted list the highest
priority, e.g., priority 9. Process 700 may assign the nets in the next
top 0.05 percent of the sorted list the next highest priority, e.g.,
priority 8. Process 700 may assign the nets in the next top 0.05 percent
of the sorted list the next highest priority, e.g., priority 7. Process
700 may assign priorities 9, 8, 7, 6, 5, and 4 in a similar example
manner for the top 3 percent of the sorted list for the short nets.
Process 700 may similarly assign priorities 9, 8, 7, 6, 5, and 4 in a
similar example manner for the top 3 percent of the sorted list for the
medium and long nets. The remaining nets in each category may be assigned
other priorities based on other percentages or other criteria.

[0092] This example way of assigning priorities by percentages is only
described for the clarity of the disclosure and not as a limitation on
the invention. Many other ways of assigning priorities using the sorted
order within categories of nets will be apparent from this disclosure to
those of ordinary skill in the art and the same are contemplated within
the scope of the invention.

[0093] Process 700 determines whether more categories of nets remain to be
sorted and prioritized in this manner (step 714). If more categories of
nets remain to be sorted and prioritized in this manner ("Yes" path of
step 714), process 700 returns to step 704 to select another category. If
no more categories remain ("No" path of step 714), process 700 ends
thereafter.

[0094] The priorities assigned to the nets in a design are usable for
further routing in accordance with an embodiment. For example, the
prioritization can be used for performing another iteration of global
routing, should an implementation so decide. The prioritization can also
be used for detailed routing. A router, whether during global routing or
detailed routing, routes a net according to the associated priority. For
example, a router may route all priority 9 nets before routing priority 8
nets, and so on.

[0095] The flowchart and block diagrams in the Figures illustrate the
architecture, functionality, and operation of possible implementations of
systems, methods, and computer program products according to various
embodiments of the present invention. In this regard, each block in the
flowchart or block diagrams may represent a module, segment, or portion
of code, which comprises one or more executable instructions for
implementing the specified logical function(s). It should also be noted
that, in some alternative implementations, the functions noted in the
block may occur out of the order noted in the figures. For example, two
blocks shown in succession may, in fact, be executed substantially
concurrently, or the blocks may sometimes be executed in the reverse
order, depending upon the functionality involved. It will also be noted
that each block of the block diagrams and/or flowchart illustration, and
combinations of blocks in the block diagrams and/or flowchart
illustration, can be implemented by special purpose hardware-based
systems that perform the specified functions or acts, or combinations of
special purpose hardware and computer instructions.

[0096] Thus, a computer implemented method, system, and computer program
product are provided in the illustrative embodiments for timing driven
routing in integrated circuit design. Using an embodiment of the
invention in an example implementation, the Steiner optimized timing
metric of -0.17 was observed to improve to -0.16 as compared to -9.73
without using an embodiment. For that example routing, the overall figure
of merit of the design deteriorated from -5.4 after Steiner optimization
to--only -28 as compared to -16422 observed without using an embodiment.

[0097] An embodiment may improve design closure repeatability and
stability. An embodiment may also improve overall timing characteristics
of the design. Using an embodiment, the time consumed in the detailed
routing phase of a design process can be significantly reduced. In one
example implementation using an embodiment caused the total time for
routing to increase from fifteen hours and seventeen minutes to sixteen
hours and fifty eight minutes (due to more iterations of global routing),
but reduced the time consumed in detailed routing from fifteen hours and
nineteen minutes to ten hours and eight minutes.

[0098] As will be appreciated by one skilled in the art, aspects of the
present invention may be embodied as a system, method, or computer
program product. Accordingly, aspects of the present invention may take
the form of an entirely hardware embodiment, an entirely software
embodiment (including firmware, resident software, micro-code, etc.) or
an embodiment combining software and hardware aspects that may all
generally be referred to herein as a "circuit," "module" or "system."
Furthermore, aspects of the present invention may take the form of a
computer program product embodied in one or more computer readable
storage device(s) or computer readable media having computer readable
program code embodied thereon.

[0099] Any combination of one or more computer readable storage device(s)
or computer readable media may be utilized. The computer readable medium
may be a computer readable signal medium or a computer readable storage
medium. A computer readable storage device may be, for example, but not
limited to, an electronic, magnetic, optical, electromagnetic, infrared,
or semiconductor system, apparatus, or device, or any suitable
combination of the foregoing. More specific examples (a non-exhaustive
list) of the computer readable storage device would include the
following: an electrical connection having one or more wires, a portable
computer diskette, a hard disk, a random access memory (RAM), a read-only
memory (ROM), an erasable programmable read-only memory (EPROM or Flash
memory), an optical fiber, a portable compact disc read-only memory
(CD-ROM), an optical storage device, a magnetic storage device, or any
suitable combination of the foregoing. In the context of this document, a
computer readable storage device may be any tangible device or medium
that can contain, or store a program for use by or in connection with an
instruction execution system, apparatus, or device.

[0100] Program code embodied on a computer readable storage device or
computer readable medium may be transmitted using any appropriate medium,
including but not limited to wireless, wireline, optical fiber cable, RF,
etc., or any suitable combination of the foregoing.

[0101] Computer program code for carrying out operations for aspects of
the present invention may be written in any combination of one or more
programming languages, including an object oriented programming language
such as Java, Smalltalk, C++ or the like and conventional procedural
programming languages, such as the "C" programming language or similar
programming languages. The program code may execute entirely on the
user's computer, partly on the user's computer, as a stand-alone software
package, partly on the user's computer and partly on a remote computer or
entirely on the remote computer or server. In the latter scenario, the
remote computer may be connected to the user's computer through any type
of network, including a local area network (LAN) or a wide area network
(WAN), or the connection may be made to an external computer (for
example, through the Internet using an Internet Service Provider).

[0102] Aspects of the present invention are described herein with
reference to flowchart illustrations and/or block diagrams of methods,
apparatus (systems) and computer program products according to
embodiments of the invention. It will be understood that each block of
the flowchart illustrations and/or block diagrams, and combinations of
blocks in the flowchart illustrations and/or block diagrams, can be
implemented by computer program instructions. These computer program
instructions may be provided to one or more processors of one or more
general purpose computers, special purpose computers, or other
programmable data processing apparatuses to produce a machine, such that
the instructions, which execute via the one or more processors of the
computers or other programmable data processing apparatuses, create means
for implementing the functions/acts specified in the flowchart and/or
block diagram block or blocks.

[0103] These computer program instructions may also be stored in one or
more computer readable storage devices or computer readable media that
can direct one or more computers, one or more other programmable data
processing apparatuses, or one or more other devices to function in a
particular manner, such that the instructions stored in the one or more
computer readable storage devices or computer readable medium produce an
article of manufacture including instructions which implement the
function/act specified in the flowchart and/or block diagram block or
blocks.

[0104] The computer program instructions may also be loaded onto one or
more computers, one or more other programmable data processing
apparatuses, or one or more other devices to cause a series of
operational steps to be performed on the one or more computers, one or
more other programmable data processing apparatuses, or one or more other
devices to produce a computer implemented process such that the
instructions which execute on the one or more computers, one or more
other programmable data processing apparatuses, or one or more other
devices provide processes for implementing the functions/acts specified
in the flowchart and/or block diagram block or blocks.

[0105] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of the
invention. As used herein, the singular forms "a", "an" and "the" are
intended to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms
"comprises" and/or "comprising," when used in this specification, specify
the presence of stated features, integers, steps, operations, elements,
and/or components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements, components,
and/or groups thereof.

[0106] The corresponding structures, materials, acts, and equivalents of
all means or step plus function elements in the claims below are intended
to include any structure, material, or act for performing the function in
combination with other claimed elements as specifically claimed. The
description of the present invention has been presented for purposes of
illustration and description, but is not intended to be exhaustive or
limited to the invention in the form disclosed. Many modifications and
variations will be apparent to those of ordinary skill in the art without
departing from the scope and spirit of the invention. The embodiment was
chosen and described in order to best explain the principles of the
invention and the practical application, and to enable others of ordinary
skill in the art to understand the invention for various embodiments with
various modifications as are suited to the particular use contemplated.